1. Field of the Invention
The present invention relates to a charge signal converting amplifier which converts a charge signal outputted from a charge generating sensor into a voltage.
2. Description of the Related Art
A charge generating sensor such as a piezoelectric element generates a charge in proportion to a load magnitude which is mechanically applied. Generally, the charge generating sensor is suitable for measuring a continuous dynamic pressure, and is used as a pressure sensor (cylinder pressure sensor) for measuring a combustion pressure in a cylinder. In order to extract a signal from the charge generating sensor, the signal from the charge generating sensor is generally converted into a voltage signal by using an amplifier with an ultra-high input impedance. Referring to FIG. 8, the charge signal converting amplifier (so-called “a charge amplifier”) has a feedback capacitor C between an input and an output of an amplifier AO having an infinite gain with an opposite phase (reverse phase). The charge amplifier is frequency used.
However, the measurement with the charge generating sensor always has a zero point drifting problem due to a charge leakage or a temperature change. For example, upon measuring the pressure by connecting the charge signal converting amplifier to a pressure sensor such as the piezoelectric element, the pressure increases from a zero level and returns thereto. Then, the zero level outputted from the charge signal converting amplifier drifts in the negative direction because the charges of the pressure sensor become negative in accordance with the charge leakages, or drift and fluctuate in the positive direction due to the temperature increase. This impedes the accurate measurement.
A description is given about examples of level fluctuations of zero points due to the charge leakages or temperature changes when the charge amplifier is connected to the cylinder pressure sensor comprising the piezoelectric element attached to a combustion chamber of the engine for measuring a combustion pressure in the cylinder.
In a normal engine having a four-stroke cycle (intake compression—combustion—exhaust), a piston reaches nearly a top dead center (TDC) position. Then, an exhaust valve is closed, and an intake valve is opened. In the case of a natural aspiration engine, the cylinder pressure becomes the atmospheric pressure. In the case of an engine with a supercharger, the pressure becomes a pressure obtained by adding a boost pressure (e.g., 500 mmHg to 1,500 mmHg) to the atmospheric pressure.
Then, the piezoelectric effect of the sensor element generates a charge proportional to a cylinder pressure load. The generated charge in this case is designated by a reference symbol −q. The charge −q is charged by the feedback capacitor C of the charge amplifier, and it is converted into a voltage signal +V by the amplifier A0, and the converted signal is outputted. Therefore, the signal level is at the zero level when the cylinder pressure becomes the atmospheric pressure. When the boost pressure exists, the signal level is at the level obtained by adding the boost pressure as a DC voltage component to the zero level of the atmospheric pressure. Then, the signal level becomes a basic level of a combustion waveform which rises by the combustion pressure to be generated later.
During the period when the piston moves from the TDC position to a bottom dead center (BDC) position, the intake continues in the meantime and, the cylinder pressure is not highly changed and is maintained to approximately the basic level. Next, the piston approximately reaches the BDC position, and the intake valve is closed. Then, the compression starts for a period from the BDC to the TDC. Simultaneously with the compression start, the cylinder pressure starts to increase, the charges of the piezoelectric element increase and are sequentially charged by the feedback capacitor C of the charge amplifier. Further, the voltage signal +V, which is converted and outputted by the amplifier AO, increases.
As a result of an ignition just before the piston reaches the TDC position (i.e., just before the maximum level of the compression pressure), the combustion pressure is generated and the generation of the combustion pressure rapidly increases the charges of the piezoelectric element. Further, the voltage signal +V, which is converted and outputted by the amplifier AO, also rapidly increases. Then, a signal outputted as the combustion pressure is the signal at the above-mentioned basic level. That is, the combustion pressure is the signal at the atmospheric pressure level in the case of the natural aspiration engine, while it is the signal at the level obtained by superimposing (or overlapping) the signal to the DC voltage component of the boost pressure in the case of the supercharged engine.
Next, after the cylinder pressure becomes a maximum, the piston moves from the TDC position to the BDC position. Accordingly, the cylinder pressure decreases. Then, the charges are inverted in their polarity, and the feedback to the piezoelectric element starts. That is, with respect to the piezoelectric element, it seems that the combustion pressure acts as a positive stress (compression force) and then the charges with the polarity of −q are charged in proportion to the stress. Further, when the combustion pressure decreases, it acts as a negative stress (tension) to the piezoelectric element, and then the polarity of the charges is inverted to +q in proportion to the stress. This phenomenon inverts the polarity of the feedback capacitor C and consequently inverts the polarity of the output signal.
After that, the piston approximately reaches the BDC position, and the exhaust valve is opened (the intake valve is still closed). Then the combustion gas is exhausted while the piston approaches the TDC position. Then, in the case of the natrual aspiration engine, the cylinder pressure of the natural aspiration engine returns to the atmospheric pressure. In the case of the supercharge engine, the cylinder pressure of the supercharged engine returns to the boost pressure. After one combustion cycle ends, the signal levels return to the level before starting the combustion cycle.
An ordinate in an oscillograph denotes the signal voltage and an abscissa denotes a rotational angle of the crankshaft or a period of time. Then, the oscillograph draws the change in the cylinder pressure during one cycle, namely, the combustion waveform is drawn. In the case of the natural aspiration engine, the combustion waveform rises from the atmospheric pressure (signal level), then, simultaneously to the combustion end, and it returns to the original signal level. In the case of the supercharged engine, the signal level rises from the boost pressure level (DC voltage level), then, simultaneously to the end of the combustion, and it returns to the original boost pressure level.
However, in the actual electric circuit comprising the sensor and the amplifier which are connected thereto, the insulation resistance is not actually infinite. Therefore the charges are leaked at each cycle in a combustion cycle having rapid repetitions such as the engine combustion, and the leaked charges are converted into the negative signal voltage. Further, upon measuring the engine combustion pressure, the pressure is measured under the circumstances of having a rapid temperature change and therefore the temperature change is superposed as the output. It seems that the pressure signal is placed on the DC current. Thus, referring to FIG. 9, a drift DV of the signal level is generated for an effective combustion pressure ECP, resulting in the offset of the waveform rising point.
Among waveform data including the above-mentioned offset of the signal level, the combustion waveform in one cycle is picked up and processed from the continuous combustion waveforms under the circumstances using a high speed and large capacity calculating apparatus in the research and development stages. The combustion waveform on the atmospheric pressure or the absolute pressure is estimated and the waveform may be analyzed. However, there are problems to be solved for application to mass production vehicles.
That is, in order to provide a system for measuring the engine combustion pressure and the combustion waveform for mass production vehicles, an offset value is calculated by an on-board device and the signal level needs to be obtained for the drift of the generated signal level. Consequently, a numerous processing capacity must be added to the on-board device, and the device increases in size and the costs increase.
As means for solving the above-mentioned problems, Japanese Patent Application No. 3123798 suggests a technology for substantially maintaining the zero level by connecting a filter having a threshold of 0.01 to 1.0 Hz to the output terminal of the charge signal converting amplifier, and by removing low frequency components of the pressure waveform. However, according to the suggested technology, the low frequency components of the combustion waveform are removed by a high pass filter and therefore the suggested technology uses substantially an AC coupling. As a result, the entire DC components of the waveform are removed. In order to achieve an accurate waveform analysis, a correction needs to compensate for the removed DC components, and thus, the calculating load necessary for the signal processing is not reduced.